highpowerZONE Products for the week of June 28, 2010
Linear Technology Says…
LTM4611: Ultralow VIN Dc-Dc µModule Regulator with Onboard 5 V MOSFET
Gate Drive Delivers 15A from Inputs Down to 1.5 VIN
Linear Technology Corporation introduced the LTM4611, a complete switch mode DC/DC uModule point-of-load regulator capable of generating its own 5V N-channel MOSFET gate drive for very efficient voltage conversion from 3.3VIN or less to loads as low as 0.8V at up to 15A. Alternative traditional low input voltage high power switch mode DC/DC converters with N-channel MOSFETs require a second regulator circuit to provide the 5V gate drive needed for logic level MOSFETs. Conversion efficiencies for the LTM4611 at full load (15A) are 92% for 3.3VIN to 2.5VOUT, 88% for 2.5VIN to 1.5VOUT, and 81% for 1.5VIN to 0.9VOUT. Applications include systems with only 3.3V, 2.8V or 2.5V as the main power rail such as compact data storage and RAID systems, ATCA and optical networking cards, as well as medical and industrial equipment.
The LTM4611 includes the main DC/DC power circuit and the gate drive circuit plus inductors, MOSFETs, input and output bypass capacitors and compensation circuitry in a small 15mm x 15mm x 4.32mm LGA (Land Grid Array) package weighing only 2.6g. The device boasts ±2% total DC output error over temperature, including line and load regulation as a result of its architecture, precision reference voltage and differential remote sense amplifier-- a key specification for loads such as FPGAs and ASICs that require better than ±3% voltage accuracy for optimum performance. To increase output capability to 30A, 45A or 60A, two, three or four LTM4611 DC/DC uModule devices can be paralleled to share current.
The LTM4611 is an ultralow voltage input (down to 1.5 V – with a top end of 5.5 V) dc-dc power regulator for 15 A dc outputs. The output voltage range is from 0.8 V up to 5 V with a total maximum of ±1.5% error. The complete regulator is within the LGA module with the power stages and all the passives including some of the input and output filtering capacitance, together with MOSFET gate drive circuits.
The main power stage uses two n-channel power MOSFETs operating in a synchronous mode together with compensation circuitry and, of course, the switching inductor. A differential remote sense is employed that compensates much better for voltage drops, and therefore errors, across PCB traces.
The highest efficiencies are obtained, of course, with the smallest difference between the input and output voltages. For 5 V in and 3.3 V out, for example, efficiencies are about 94% for loads greater than 5 A. At the other extreme, with 5 V input and 1 V output, the efficiency drops about 10%. A burst-mode or pulse-skipping operation is available for light loads to increase efficiency.
Multiple LTM6411 modules can be paralleled to raise the available load current, with say 2, 3, or 4 modules to deliver 30 A, 45 A, or 60 A. External synchronization of the modules allows them to be driven with out-of-phase clocks; this reduces the output ripple and lowers the external capacitance requirements. Overcurrent foldback protection is provided, as is output overvoltage protection.
A tracking/soft-start pin allows a slave converter to sequence/track the output of a master converter so that complex loads, like FPGAs, can be powered up and/or down correctly. The operating frequency can be set by leaving the PLLFLTR/fSET pin floating (nominal 500 kHz), or connected to 0.85 V (330 kHz), or to 2.0 V (780 kHz). The external sync range – for noise sensitive applications or for multiple part parallel operations – is from 360 kHz to 710 kHz. The data sheet gives no indication of the on-resistances of the power MOSFETs.
The LTM µModule family has been a major boon for Linear. It shouldn’t have been: with only the semiconductor products in house – and that only after acquiring the power MOSFET technology to go ahead – there should have been other vendors who could have raced ahead with all the passives in house as well. But Linear has focused on just a few sizes of LGA packaging and has secured a supply chain of passives to keep well ahead of the competition. The LTM4611 is no exception to that and it will be a highly profitable major volume part with uses in many varied point-of-load applications.
The LTM4611 is in production in a thermally-enhanced 15 x 15 x 4.32 mm LGA with an operating temperature range of -40ºC to 125ºC. Pricing is $19.45 in 1000-piece lots. Demonstration kits are available.